Quick update of the Placemark Clustering project: we’ll be doing user tests using the uk police crime map later this summer (discussed below) comparing it to a chloropleth grid (translation = head map based on grid, I explain further here)

In thinking about this I’ve hunted down some examples and I thought it would be interesting to name check 3.

No Collation: The first map is Oakland Crime Spotting (bottom inset in figure) that is very similar to San Francisco Crime mapping, reviewed here. Unlike the other two maps it attempts no point collation at all, I image the authors would argue that they deal with the problem by providing sophisticated filtering tools to reduce the point density. However, it doesn’t help if the user wants to get an overview picture of crime across the area the map covers.

Traditional Choropleth: Switching to the the UK, the Metropolitan Police (=London for non UK readers) offer a choropleth map based on wards and subwards (top left insert). I regard this as the traditional approach. Notably it doesn’t show actual figures for postcodes, only sub wards – a sub ward is a collection of postcodes. My problem with this is that almost no one knows the boundaries of wards and sub wards so its a strange way to split the city up. (Aside: in my experience, Londoners tend to split London up based on tube stations)

Point Collation: The UK police offer a national map which uses point collation (top right insert). This is the main one we’re planning to test as IMHO it isn’t an effective way to visualise the data (related post). It offers a finer grain of data – you don’t actually see the true location of the crime but it is collated down to the postcode level. In London, a postcode is roughly equal to a single street.